Myelin integrity is an especially promising target for manipulation, because, unlike the volume of grey matter, it changes throughout life. That it can change may seem surprising given its heritability. One explanation is that genes drive us to interact with our environment in ways that can lead to changes in myelin integrity, says Thompson.

Identifying the genes that promote high-integrity myelin could lead to ways to enhance the genes' activity or artificially add the proteins they code for. This may in turn provide therapies for multiple sclerosis, autism and attention deficit disorder, which are associated with degraded myelin. Intelligence enhancement in people who just want help passing an exam, say, is also "within the realm of possibility", Thompson reckons.

Medical treatments are still a long way off, warns Naomi Friedman, a behavioural geneticist at the University of Colorado in Boulder: "There'll be interactions between genes and environment that are going to have to be disentangled."